This invention relates in general to cast vehicle wheels and in particular to a mold for casting a vehicle wheel, and a method for casting a vehicle wheel by use of the mold.
Cast wheels formed from light weight metal alloys are replacing steel wheels on an increasing number of vehicles. Such cast wheels provide both a reduction in weight from steel wheels and an attractive appearance. Cast wheels are cast by introducing a molten metal, typically an alloy of a light weight metal, such as aluminum, magnesium or titanium, into a wheel mold to form a wheel casting. The wheel casting is allowed to cool to solidify the metal, and the solidified wheel casting is removed from the mold.
A traditional wheel mold includes a base member that supports the other pieces of the mold. Two or more retractable side members are carried by the base member. A removable cup-shaped top member is disposed within the side members. Upon assembly, the base, top and side members cooperate to define a mold cavity wherein the wheel casting is cast. The mold cavity includes an annular rim cavity for casting the wheel rim and a disc cavity for casting the wheel disc. The rim cavity has a uniform radial width around the circumference of the rim cavity.
The traditional wheel mold, having a rim cavity with a uniform radial width, is not always effective in solidifying the molten metal forming the wheel rim. As the molten metal solidifies, a crystalline structure consisting of individual metal grains is formed in the wheel rim. If the molten metal solidifies too slowly, a large grain structure is formed that is relatively weak and, porous compared to the smaller grain structures formed during rapid solidification. The use of a rim cavity having a uniform radial width can result in slow crystallization of portions of the wheel rim, causing these portions to be relatively weak and porous. A wheel rim having weak portions is unsuitable for use on a vehicle, so that the wheel rim must be scrapped. High porosity of the wheel rim can allow escape of the pressurized air contained in a tire mounted on the finished wheel. In view of the problems associated with the traditional wheel mold, it would be desirable to provide a mold for casting a vehicle wheel that is more effective in solidifying the wheel rim, and a method for casting a vehicle wheel by use of such a mold.
The present invention relates to a mold for casting a vehicle wheel, including a plurality of mold members that cooperate to define a mold cavity. The mold cavity includes an annular rim cavity for casting a wheel rim and a disc cavity for casting a wheel disc. The mold members define a central axis and have a passageway for introducing molten metal into the rim cavity. The rim cavity has a radial width that changes in a circumferential direction between a maximum width portion of the rim cavity located proximally relative to the passageway, and a minimum width portion of the rim cavity located distally relative to the passageway.
The invention also relates to a method for casting a vehicle wheel. A mold as described above is provided. Molten metal is introduced through the passageway into the rim cavity to fill the mold cavity and form a wheel casting. The wheel casting is allowed to cool sufficiently to solidify the metal. The changing width of the rim cavity causes a thermal gradient in the molten metal such that solidification proceeds in a circumferential direction from the minimum width portion of the rim cavity to the maximum width portion of the rim cavity. The solidified wheel casting is removed from the mold.
Various objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings.